A recovery probe is sent into space to recover a missing Mars Probe, but the recovery goes wrong, and a strange message is received on Earth.
Season seven, Jon Pertwee's first, I see as a true purple patch in the show's history, many look at Spearhead, Silurians and Inferno as out and out classics, they are, and whilst for many, Ambassadors isn't quite in that league, I'd argue it isn't that far off the pace. Pertwee needed no time at all to settle into the role, he's as good here and as much in character as he is in Planet of Spiders.
Liz is by far the most underrated companion, getting only one series, the character was considered too clever, but Caroline John was outstanding, formidable, and up to The Doctor. I loved the scenes of The Doctor and Liz hopping about in time.
It has a real sixties sci fi thriller vibe, with a mix of The Quatermass Experiment thrown in. Lots of action and gunfights, scenes that fans would get used to during Pertwee's time.
I think it's a very good start, great story, lots of action, plenty of sci fi, and most of all, lots of intrigue. 9/10